Despicable Dark Relics: generated by gravity with unconstrained masses

TL;DR

This paper introduces a gravitational mechanism during reheating that can produce a wide range of dark relics with masses from keV to 10^8 GeV, which are undetectable by current non-gravitational searches.

Contribution

It demonstrates a generic, efficient gravitational process for dark relic production during reheating, applicable for any inert scalar with non-minimal curvature coupling.

Findings

Relic abundance can match observed dark matter for various masses.

Production mechanism is efficient for natural coupling values around unity.

Relics are undetectable by current non-gravitational searches.

Abstract

We demonstrate the existence of a generic, efficient and purely gravitational channel producing a significant abundance of dark relics during reheating after the end of inflation. The mechanism is present for any inert scalar with the non-minimal curvature coupling $\xi R\chi^2$ and the relic production is efficient for natural values $\xi = {\cal O}(1)$. The observed dark matter abundance can be reached for a broad range of relic masses extending from $m \sim 1 {\rm k eV}$ to $m \sim 10^{8} {\rm GeV}$, depending on the scale of inflation and the dark sector couplings. Frustratingly, such relics escape direct, indirect and collider searches since no non-gravitational couplings to visible matter are needed.